Pine, say, about Henry's number (5)
I believe the answer is:
three
'number' is the definition.
(three is a number)
'pine say about henry's' is the wordplay.
'pine say' becomes 'tree' (pine is a kind of tree).
'about' means one lot of letters goes inside another.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'tree' placed around 'h' is 'THREE'.
(Other definitions for three that I've seen before include "Number constituting a crowd?" , "Square root of nine" , "Number of strikes before you're out" , "A crowd, you might say" , "Feet in 3 Down" .)